1. Field of the Invention
The present invention generally relates to a display device and more particularly relates to a display device in which two electrodes, arranged to face each other with a display medium layer interposed between them, include conductive layers with mutually different work functions.
2. Description of the Related Art
Various types of office automation (OA) equipment, such as personal computers with displays, have rapidly reduced-their sizes and weights so significantly these days as to carry or move them to anywhere we like. But their manufacturing costs have not been successfully decreased as fast, or as significantly, as their sizes and weights. Accordingly, under the current circumstances, it is one of the most important and most pressing tasks to reduce the manufacturing cost of a display device.
A display device normally has a configuration in which a pair of electrodes is arranged so as to face each other with a display medium layer, exhibiting electrooptical properties, interposed between them. Such a display device conducts a display operation by applying a voltage to the display medium layer (i.e., creating a potential difference between the electrodes). The display medium layer may be made of a liquid crystal material, an electroluminescent material, a plasma or an electrochromic material, for example. Among other things, liquid crystal displays (LCDs), using a liquid crystal material for the display medium layer, have been popularized faster and more extensively than any other type of display device, because LCDs can conduct a display operation at relatively low power consumption.
Recently, however, demand for LCDs with even lower power dissipation has been escalating. To meet such demand, reflective LCDs using external light for display purposes have been researched and developed more and more extensively as replacements for transmissive LCDs that usually need a backlight.
Reflective LCDs are currently used in numerous types of mobile telecommunications units including cell phones. Meanwhile, reflective LCDs with a secondary light source, contributing to allowing the user to perceive a displayed image under any environment, have also been developed. Such a reflective LCD with a secondary light source may have one of the following two configurations.
One of the two possible configurations utilizes a front light method in which incoming light, which has been laterally incident onto the side surfaces of the reflective LCD, are uniformly introduced into the reflective LCD by way of a light guiding member. Specifically, in this configuration, the light guiding member is provided on the frontmost surface of the LCD (i.e., closest to the viewer) and a light source such as a cold cathode tube or an array of LEDs, which is normally used as a backlight, is provided on the right- and left-hand sides of the LCD.
In the other configuration, each of multiple pixel electrodes included in a reflective LCD is provided with a transparent electrode region and a backlight is provided on the back surface of the display (i.e., on the other side of the display opposite to the viewer (or front) side). An LCD having such a configuration can perform both the function of a reflective LCD and that of a transmissive LCD, and is sometimes termed a “semi-transmissive” type.
In each of the reflective LCDs described above, a counter electrode is provided on the surface of a counter substrate so as to face a liquid crystal layer. The counter electrode normally includes a transparent conductive layer made of ITO, for example, and an alignment film. On the other hand, on an active-matrix substrate including switching elements such as TFTs and pixel electrodes thereon, a reflective conductive layer, having a reflective function and including at least Al, for example, and another alignment film are normally provided so as to face the liquid crystal layer, too. In this case, the alignment films provided for the counter substrate and the active-matrix substrate both cover their associated conductive layers and both make contact with the liquid crystal layer. It should be noted that a member that includes a conductive layer and a polymer film and that makes direct contact with the display medium layer to apply a voltage thereto will be referred to herein as an “electrode”.
In such a reflective LCD, conductive layers with mutually different work functions are provided for the counter substrate and the active-matrix substrate. In that case, if the conductive layers having mutually different work functions are arranged on the counter substrate and the active-matrix substrate so as to face each other, then an electrode potential difference will be created between the two conductive layers due to the difference in work function as shown in FIG. 10. In such an LCD, an offset voltage is normally added to an AC voltage applied to the liquid crystal layer such that a DC voltage component, produced by the electrode potential difference, is not applied to the liquid crystal layer.
However, in such an arrangement in which the conductive layers with different work functions are provided for the counter substrate and the active-matrix substrate, even if the offset voltage is added, the DC voltage component may sometimes be added to the liquid crystal layer during the operation of the LCD.
The present inventors discovered via experiments that such a DC voltage component was produced due to optically induced deterioration of the alignment films. Specifically, when the alignment films on the pixel electrodes and on the counter electrode deteriorate optically, the apparent electrode potentials of the pixel and counter electrodes will both change, thus creating a difference between the electrode potentials of the pixel and counter electrodes. As a result, the DC voltage component is applied to the liquid crystal layer. This phenomenon occurs when the two opposed conductive layers have mutually different work functions.
When the DC voltage component is applied to the liquid crystal layer due to the creation of the electrode potential difference between the pixel and counter electrodes as described above, the brightness will change at short intervals to produce a flicker and deteriorate the display quality significantly. Also, if the DC voltage component is continuously applied to the liquid crystal layer for a long time, then the reliability of the liquid crystal material might be risked.
To understand this phenomenon as resulting from the difference between electrode potential levels, a member including the conductive layer and the alignment film will be referred to as an “electrode”. The difference in electrode potential level between two associated electrodes may be obtained by a flicker minimization method to be described later. It should be noted that the “electrode potential” of a conductive layer herein means an electrode potential that is unique to the material of the conductive layer.